on linux and windows, npm install -g seems to be very slow

(**note**: this issue pertains to doing an npm install from github, not from the prebuilt binaries stored on npmjs.com i.e. `@ibm-functions/shell`)

running `npm install -g --verbose starpit/shell` takes 2 minutes on linux and windows, but 18 seconds on macOS.

the slowness seems to come from long pauses like this:
```
npm verb correctMkdir .../.npm/_locks correctMkdir not in flight
```